This invention relates to the in situ gasification of subterranean coal deposits.
In recent years, there has been considerable interest in the development of methods for the recovery of heating values from subterranean coal deposits which are not readily mined by conventional mining methods. One method for recovering heating values from such coal reserves is in situ gasification. In general, such processes involve the partial oxidation of subterranean coal deposits by the injection of a free-oxygen containing gas to produce a product gas which is recovered from the coal seam and processed to produce heat, synthetic natural gas or a variety of other products. The product gases so recovered contain substantial heating value as well known to the art. In general, such processes have utilized an injection well and a product recovery well for the gasification of the subterranean coal deposits. Many variations and modifications of this approach have been used, including a plurality of wells, slanted wells, and the like.
In one such method, as set forth in "The Controlled Retracting Injection Point (CRIP) System: A Modified Stream Method for In Situ Coal Gasification", R. W. Hill and M. J. Shannon, Proceedings of Seventh Underground Coal Conversion Symposium, Fall Leaf Lake, Calif., Sept. 8-11, 1981, a horizontal borehole was used for the injection of free-oxygen containing gas. In general, the method disclosed comprises the use of a horizontal borehole containing an injection tubing through which gas is passed to a discharge point in the vicinity of a production well. The gasification of the subterranean coal deposit occurs in the vicinity of the free-oxygen containing gas discharge point to produce product gases which are recovered through the production well. When the portion of the coal deposit in the vicinity of the free-oxygen containing gas outlet has been consumed, the tubing is severed at a point so that an additional portion of the coal deposit can be gasified. The tubing is severed by the use of wire-line tools, explosives, pyrophoric materials, or the like.
Other references which were considered in the preparation of the present application are as follows: LLNL Underground Coal Gasification Project: Quarterly Progress Report--October through December, 1980, Editors D. U. Olness and Wallace Clements, Jan. 26, 1981, UCRL-50026-80-4; and "Elf Drills 1000+Feet Horizontally", Andre P. Jourdan and Guy A. Baron, Petroleum Engineer International, September, 1981, pp. 51-58.
As a result of the continuing interest in the in situ gasification of subterranean coal deposits, a continuing search has been directed to the development of improved methods whereby such coal can be gasified more economically and and reliably.